1. Field of the Invention
The present invention relates to surfaces used to reflect and transmit light, and particularly to materials that reflect and transmit even diffusion of light energy from or through their surfaces.
2. Description of Related Art
Special light reflectant surfaces are used in a variety of applications requiring light energy to be close to completely reflected while providing an even distribution of light from the surface. While good mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle of light contact. For many applications it is important that light be reflected with an even distribution of light from the surface. This latter property is referred to as diffuse or "lambertian" reflectance. For instance, projection screens, such as those used for slide or motion picture presentations, must provide both high reflectivity and a light diffusion/distribution over a sufficiently wide field so as to provide a clear image to most of an audience. Many reflectant screens employ a coating of glass beads or similar material as a reflectant aid to provide excellent reflectivity over a defined projection field (e.g., approximately 20.degree. from a center line), with significantly diminished reflectivity outside of the defined projection field. These screens provide very good viewing within the defined field, and are less prone to interference from stray light sources other than the primary light source aimed perpendicular to the screen. In order to provide better viewing to a wider defined field, matte-finished screens are effective at providing a more even light distribution to an entire audience. Although hardly critical for most projection screen applications in darkened rooms, in either instance it is important that the screen absorb or transmit as little light as possible so as to assure maximum reflective image to the audience.
Reflectivity is far more critical in many other applications. For instance, displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs), etc.), whether relying on supplemental lights (e.g., backlight) or merely ambient light, require very good diffuse reflectant back surfaces to maximize image quality. Reflectivity is particularly critical with backlighted displays in battery powered equipment, where better reflectivity is directly related to smaller required light sources and resulting lower power demands.
Even more demanding applications for highly reflective materials are in casings used in laser or optical test equipment construction. Since the efficiency of such equipment is directly dependent upon its ability to effectively process light energy, it is critical that the equipment be constructed with material that has extremely high reflectivity and excellent diffusion properties.
Contrastly, there are a variety of applications where it is preferred to provide effective light transmittance through a material. Examples of these applications include some diffusion filters, rear projection screens, transflective displays, etc. With most materials which reflect and transmit light there is also an absorption component. In cases where both reflectance and transmittance are needed concurrently, absorption of light energy is undesired because it results in wasted light energy.
In the case of rear projection screens, the screen is positioned in between the light source and the audience. These screens trade-off reflectance for transmittance while maintaining a high level of diffusivity. Once again, to maximize the light energy output, it is important that the screen material absorb the lowest amount of light possible.
Another application where a trade off of reflectance for transmittance is desired is in certain avionic transflective LCD displays. These transflective (both reflective and transmittive) displays employ a reflector that supplies light to an LCD display using ambient light. In this case, where ambient light is insufficient to light the LCD display, a backlight is used. This backlight supplies light through the reflector; hence, the reflector must have both reflectance and transmittance properties. Under these conditions, both diffuse reflectance and diffuse transmittance are desired while absorption of light is not desired.
Another application of where diffuse reflective materials are employed is as a diffuser in solar collectors or photovoltaic cells. Since solar-driven devices such as these generally use sunlight in the range of 300 to 2200 nm, the diffuse nature and low absorption of the present invention are particularly suitable. As is described in U.S. Pat. No. 4,571,448, issued to A. M. Barnett, a photovoltaic cell can have increased efficiency by providing a diffuse textured reflective back surface. In these applications, it would appear desirable to provide a material that can further increase reflection efficiency over the entire solar spectrum of 300 to 2200 nm.
Due to the many different applications that exist for reflectant materials, it is not surprising that there are many different commercially available products with a variety of diffuse reflective properties. Until the present invention, the best material known with excellent diffuse reflectivity was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTRALON by Labsphere, Inc., North Sutton, N.H. This material comprises lightly packed granules of polytetrafluoroethylene that has a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved With this material, with reflectance over previously available reflectant material increasing from 97% to better than 99%.
Despite the reported advantages of SPECTRALON material, it is considered quite deficient in many respects. First, this product is a relatively hard block of material that must be carefully carved or machined to desired shapes and dimensions. This severely limits how and where this material can be used and greatly increases the cost of using this material in many applications, especially where non-planar shapes are desired. Therefore, where a pliable material is desired in various light reflective applications, it is clear that the SPECTRALON material is not capable of supplying such a property. Furthermore, the additional machining process provides yet another source for contamination that can be detrimental to its reflective properties.
Second, the SPECTRALON material is apparently limited, both structurally and in its ability to reflect light, to a relatively thick minimum depth (i.e., a thickness of greater than 4 mm). Again, this serves to limit where and how this material can be used. Moreover, this limitation tends needlessly to increase both the amount of material required for a given application as well as the weight of the material required for such application.
Third, the SPECTRALON material is apparently relatively expensive to manufacture and purchase. These costs are only increased by the material's difficulty in processing into the final shape from the hard form (i.e., excessive amounts of material may have to be machined away and discarded during production) and its minimum thickness requirements. As a result, the SPECTRALON material is too expensive to be used in many applications that might otherwise benefit from its reflective properties.
Fourth, although SPECTRALON has high diffuse reflective properties, it is contemplated that even better performance may be possible in this regard. For instance, the SPECTRALON material has very good reflective properties for visible light up to a near IR range (i.e., from 300 to 1800 nm), the reflectivity of this material diminishes dramatically above 1800 nm. Moreover, it is believed that even better reflective performance might be possible even in the visible light range where SPECTRALON material delivers its best performance.
Another material which is commonly used as both a diffuse reflector and transflector is barium sulfate. Barium sulfate is applied in a powder form on various substrates, such as metal or glass, to address specific reflectance or transflectance needs. While it does provide relatively good optical properties, barium sulfate is difficult to apply evenly and tends to flake off in end use applications, especially where vibration and or abrasion are present.
It is accordingly a primary purpose of the present invention to provide an improved highly light diffuse material and method of use that can perform as well or better than existing diffuse reflective or transflective materials and has better handling characteristics.
This and other purposes of the present invention will become evident from review of the following specification.